Brood sex ratios, female harem status and resources for nestling provisioning in the great reed warbler (Acrocephalus arundinaceus) |
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Authors: | H Westerdahl Staffan Bensch Bengt Hansson Dennis Hasselquist Torbjörn von Schantz |
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Institution: | (1) Department of Animal Ecology, Ecology Building, Lund University, 22362 Lund, Sweden e-mail: helena.westerdahl@zooekol.lu.se Fax: +46-46-2224716, SE |
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Abstract: | The theory of parental investment and brood sex ratio manipulation predicts that parents should invest in the more costly
sex during conditions when resources are abundant. In the polygynous great reed warbler, Acrocephalus arundinaceus, females of primary harem status have more resources for nestling provisioning than secondary females, because polygynous
males predominantly assist the primary female whereas the secondary female has to feed her young alone. Sons weigh significantly
more than daughters, and are hence likely to be the more costly sex. In the present study, we measured the brood sex ratio
when the chicks were 9 days old, i.e. the fledging sex ratio. As expected from theory, we found that female great reed warblers
of primary status had a higher proportion of sons in their broods than females of lower (secondary) harem status. This pattern
is in accordance with the results from two other species of marsh-nesting polygynous birds, the oriental reed warbler, Acrocephalus orientalis, and the yellow-headed blackbird Xanthocephalus xanthocephalus. As in the oriental reed warbler, we found that great reed warbler males increased their share of parental care as the proportion
of sons in the brood increased. We did not find any difference in fitness of sons and daughters raised in primary and secondary
nests. The occurrence of adaptive sex ratio manipulations in birds has been questioned, and it is therefore important that
three studies of polygynous bird species, including our own, have demonstrated the same pattern of a male-biased offspring
sex ratio in primary compared with secondary nests.
Received: 1 June 1999 / Received in revised form: 10 January 2000 / Accepted: 12 February 2000 |
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Keywords: | Sex ratio manipulation Polygyny Female harem status Resources for nestling provisioning |
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